/Engine/Plugins/Online/OnlineServices/Source/OnlineServicesInterface/Public/Online/OnlineError.h
Implementations of this would contain any contextual information about the error (eg, if anFTextorFStringexposed some fields to be filled in at run time), and likely the platform specific error representation (eg, NSError on apple that we can use to get a string to display to user and a string to log)
class IOnlineErrorDetails